In natural aqueous environments, man-made interfaces are subjected to accumulation of unwanted biological fouling or biofouling. It is difficult to accurately quantify early-stage development of biofouling, particularly in situ, because they are typically composed of diverse groups of microscopic organisms and other organic material that form heterogeneous, soft, and often transparent structures.
Biofouling forms when organic matter (e.g., proteins, sugars, nucleic acids, lipids) and microorganisms settle on a surface and discharge a sticky matrix of polymeric substances that protect them and eventually attract or trap more or larger, multicellular fouling organisms (e.g., barnacles, mussels, algae). Biofouling may be harmful even in the earliest stages: thin layers of biofouling on medical implants routinely lead to full-fledged infections, and an increase in roughness of a ship's hull by as little as 10 μm can increase drag and affect fuel efficiency. Furthermore, fouling inhibits flow through industrial filters, exacerbates corrosion, reduces heat transfer efficiency, persists in water distribution networks, and otherwise permeates the built environment with deleterious effects. Often, fouling occurs in places that are not suited to traditional sanitary laboratory testing so quantifying biofouling growth in the environment is a challenge.
There are several American Society for Testing and Materials (ASTM) standards for the assessment of biofouling on marine antifouling coatings. However, these standards have limited applicability, require long-term data collection (up to 2 years), and are only semi-quantitative because they rely on subjective estimates of areal coverage based on visual inspection and on counting organisms of various fouling species (e.g., barnacles, oysters/mussels, tubeworms, algae, etc.). Since the methods are based on visual inspection, it is typically not possible to quantitatively evaluate the development of early-stage fouling. Subtle differences in this soft, transparent or semi-transparent, heterogeneous film of microorganisms cannot be distinguished with the naked eye, yet may serve as an important predictor of the development of fouling in the long term. Additionally, counting species does not correlate with the mass or volume of organic material present as the size of each individual organism can vary greatly depending in part upon the stage of development or age of the organism. Moreover, the conditioning film, biofilm, or slime layer often covers the full area of a sample surface but it typically does not do so evenly, so areal coverage can be a misleading measurement that may not accurately represent the progress of fouling development.
By contrast, there is a strong collection of ASTM standards for the evaluation of biofouling formation in laboratory settings with well-controlled exposure to a limited selection of microorganisms. These test methods serve as a model for improved quantification of marine biofouling; however, they are for laboratory evaluation and use specialized bioreactors. As a result, these methods are typically not suited to field evaluation and can be time and labor intensive. Additionally, the reactors have a limited range of flow rates (from static to ˜7 mL min−1), hold a limited number of samples, and cannot easily be adapted for use with multispecies biofouling communities.
Optical biosensors have been employed to analyze biofouling formation. These techniques offer a high level of detail such that individual bacterial cells can be seen at high magnifications; however, the utility of optical images is often limited by a narrow field of view that cannot capture the heterogeneous and topologically diverse nature of bacterial biofouling. In addition, some techniques can reconstruct biomass distribution only for biofouling of a limited thickness and opacity.
At least some of the embodiments of the disclosure described below are directed to apparatus and methods for quantifying an amount of biological material upon a surface.